Central to plant biology are the processes which govern allocation of photosynthetic products among growing tissues. Recent findings indicate that distribution and metabolism of these resources may be regulated in part by short-term alterations of gene expression in response to carbohydrate availability. Sucrose synthase is the primary enzyme responsible for sucrose hydrolysis in many developing plant tissues which depend on imported sucrose. This laboratory has shown that expression of a gene encoding this enzyme in maize root tips is modulated by the level of sugars present. This research goals are to: 1) characterize the role of carbohydrate status in regulation of sucrose synthase (Sh1) gene expression, 2) define the physiological consequences of rapid changes in Sh1 expression, and 3) identify other carbohydrate regulated genes. The approach will combine metabolic studies with quantification of gene expression using gene specific DNA probes and antibodies. The conditions under which this response occurs in vivo will be established. In addition, maize sucrose synthase and invertase mutants will be used to assess the metabolic effects of short- term changes in sucrose synthase gene expression. Finally, the possiblity that this response includes other metabolically important genes in addition to sucrose synthase will be addressed. This research will contribute not only to the clarification of the interplay between gene expression and metabolism, but also to elucidation of mechanisms controlling sucrose allocation in higher plants. Perhaps the first example of sugar-modulated expression of an identified plant gene has been described by this laboratory. Specifically, the Sh1 form of sucrose synthase is rapidly and selectively induced in maize root tips depleted of sucrose. Sucrose is the predominant form in which sugars are redistributed in most species, therefore its synthesis and breakdown are crucial to the plant. Sucrose synthase catalyzes a reversible cleavage of sucrose, and is thus vital in tissues which import and metabolize sucrose. In many instances, sucrose synthase activity is positively correlated with the extent of sucrose movement into a given tissue. Therefore, the interaction of sugar levels with sucrose synthase gene expression is a potential mechanism for regulation of sucrose import and utilization. This research explores sugar regulation of sucrose synthase expression and its metabolic consequences, in detail, at the molecular, cellular, and whole plant levels.
